1. Field of the Invention
The present invention relates to a semiconductor device applied to a load drive circuit having a configuration wherein a switching element is interposed between a positive electrode of a power supply and a load, and in particular, relates to a semiconductor device such that the switching element is a protected in a state wherein the power supply is connected in a reverse orientation.
2. Description of Related Art
A vehicle power supply circuit device described in, for example, JP-A-2005-19532 has been proposed as this kind of semiconductor device. This heretofore known example includes a power chip 101 that switches a current supply from a vehicle-mounted power supply 103 to a load 104 between an on-state and an off-state, as shown in FIG. 7 and FIG. 8. The heretofore known example has an internal circuit 105 that controls the switching on and off of the power chip 101, and includes a control chip 102 configured separately from the power chip 101. A vehicle power supply control device 110 such that control of the power supply to the load 104 is possible is configured by the power chip 101 being controlled by the internal circuit 105 in a state wherein the power chip 101 and control chip 102 are connected in parallel with the vehicle-mounted power supply 103. Further, the control chip 102 includes a depression type MOSFET 107, wherein a source terminal 171 is connected to a power supply terminal 121 or a ground terminal 122 of the control chip 102, while a drain terminal 172 is connected to the internal circuit 105.
Further, the heretofore known example described in JP-A-2005-19532 is such that the depression type MOSFET 107 is connected to the vehicle-mounted power supply 103 positive electrode side of the control chip 102, the depression type MOSFET 107 functions as a constant current element when the vehicle-mounted power supply 103 is reverse-connected, voltage necessary for causing a current of the same value as the constant current flowing through the depression type MOSFET 107 to flow is applied to the internal circuit 105, and the remaining voltage is applied to the depression type MOSFET 107.
When the vehicle-mounted power supply 103 is forward-connected, the voltage drop is small owing to a low resistance region, which is a characteristic of the depression type MOSFET 107, being used. However, the heretofore known example is such that it is essential to connect a depression type MOSFET to the control chip 102, an additional manufacturing step is necessary in order to integrate a normal MOSFET that is not of a depression type and a depression type MOSFET, and there is an unresolved problem in that the number of man-hours increases, and cost increases due to a drop in yield.
Also, as no circuit generating a signal to be supplied to the gate of the power chip is shown in the internal circuit 105, there is also an unresolved problem in that trouble such as a transistor in the internal circuit 105 that should by rights be turned on when the vehicle-mounted power supply 103 is reverse-connected being turned off, and the output of an OUT terminal 125 being of high impedance, may occur. Because of this, there is an unresolved problem in that it is necessary to specify a circuit that guarantees problem-free operation even when a vehicle-mounted power supply is reverse-connected.